Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As a system including the fuel cell stack, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2005-166439 (hereinafter referred to as the conventional technique 1) is known. As shown in FIG. 13, the fuel cell system uses a solid oxide fuel cell 4 having a solid electrolyte 1, a fuel electrode 2 on one side of the solid electrolyte 1, and an air electrode 3 on the other side of the solid electrolyte 1. The air is supplied to the air electrode 3 as the oxygen-containing gas, and the fuel gas is supplied to the fuel electrode 2 for generating electricity by reaction of the fuel gas and the air.
In the fuel cell system, additionally, a start-up combustor 5, an exhaust gas combustor 6, and a heat exchanger 7 are provided. At the time of starting operation of the fuel cell system, the start-up combustor 5 reforms or imperfectly combusts the fuel gas supplied from the outside to supply the fuel gas to the fuel electrode 2 as a reducing gas. The exhaust gas combustor 6 combusts the off gas discharged form the fuel electrode 2. The heat exchanger 7 heats the air by the heat generated in the exhaust gas combustor 6.
According to the disclosure, in the structure, the large amount of unconsumed exhaust gas such as carbon monoxide produced in the fuel cell system at the time of starting operation of the fuel cell system can be reduced as much as possible, generation of heat stress due to the temperature difference can be prevented by heating both of the fuel electrode 2 and the air electrode 3, and improvement in the durability of the fuel cell system is achieved. Further, both of the fuel electrode 2 and the air electrode 3 can be heated at the same time efficiently, and the time required for starting operation of the fuel cell system is reduced.